This invention relates to devices for high frequency induction welding, and in particular to apparatus of this kind which is fluid cooled.
It is well-known to weld together plastic laminates with aluminium foil attached thereto by means of inducing a high frequency current in the foil. This technique has been developed considerably during recent years, and it is now possible to weld together most types of thermoplastic products provided that the detailed outlines which are to be welded together include some kind of eddy current creating material. This may be aluminium foil, iron oxide powder, or other similar electrically conductive material applied in the region of the intended welding area. In certain cases, the plastic material itself contains a sufficient amount of conducting pollutants for directly allowing welding together by induction welding. The operating frequencies used normally fall within the one to five MHz region. In this frequency region the resistive power loss in the welding tool (i.e. the inductive loop) is not great. This due to the high inductive reactance of the loop at said frequencies and the so-called skin effect.
Nevertheless, during the operation of the welding tool, considerable heating of the tool occurs. In order to obtain an undisturbed operation during constant use, the built up heat must be conducted away. In prior art apparatus this heat removal was accomplished by circulating a cooling medium through an induction loop comprising an electrically conducting pipe having a fluid inlet and outlet for transmission of the cooling medium. Reasons for using pipes for prior art induction coils include the simple and efficient cooling characteristics thereof, and the relatively large outside surface area thereof, resulting in lowered electrical resistance to skin effect currents.
The prior art inductive welding tools based on the known technique, require a precise working operation to bend the pipe to a loop having the required well-defined inner and outer contours. Thereafter the loop is placed in an isolating support having the corresponding, well-defined inner contour. This is a very expensive process. In spite of careful bending of the pipe, there arise restrictions at the regions of the bends of the pipe. These restrictions decrease the available cross-section for the circulation of the cooling fluid which may result in undesired effects when welding. Also, the cooling channels of the pipes are relatively inaccessable. Thus there is no simple method for cleaning the channels.